Reaction-jet helicopters use compressed air delivered through hollow-body rotor blades to induce rotational movement of the blades, unlike conventional helicopters which use a mechanical transmission system to transfer power from the engine to rotate the blades. Reaction jets, also known as pressure jets and tip jets, are nozzles located at the tip of these hollow-body blades. The air travels through the hollow blades from root to tip and exits at sonic or supersonic speed at the nozzles. The thrust generated as the air exits the nozzles generates the blades' rotational movement.
The motive air is generally delivered to the rotor blades from a compressor that is driven by the helicopter's engine, whether gas turbine or piston. Alternatively air can be bled from the compressor discharge. Reaction-jet helicopters come in three variants, cold, warm or hot cycle tip-jet systems that are differentiated on the basis of the gas temperature entering the rotor blades. The gas temperature depends on the source of the motive air. Cold-cycle tip-jet systems receive relatively cool air diverted from air exiting the compressor before it enters the burner. Hot cycle tip-jet systems receive air post-burner, which of course is hotter than the air entering the burner. Hot air delivered to the rotor blades provides more thrust than cooler air for a given amount of air.
Reaction-jet helicopter systems are much lighter than conventional helicopters because by using air to propel the rotor blades, they do not require the heavy mechanical transmission system to drive the rotor blades. Because reaction jet helicopters are lighter, the reaction jet will generally carry more cargo (freight or passengers) than a conventional helicopter, for a given gross takeoff weight that includes the fuel load. The present reaction-jet designs however are less fuel efficient than conventional helicopters and, accordingly, for a given takeoff weight will not be able to fly as far. However, if some of the cargo weight is sacrificed for a heavier engine that is significantly more fuel efficient than a conventional helicopter engine, then the reaction-jet system of the present invention can be superior to a conventional helicopter.
A recuperator is a special purpose heat exchanger positioned within the air flow of an engine that recovers waste heat from the engine, typically to increase the engine's overall efficiency. For example, in a gas turbine engine, air is compressed and mixed with fuel, which is then burned and used to drive a turbine, generating hot exhaust. A recuperator coupled to the engine between the turbine and the exhaust captures and returns some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the fuel burner stage. Since the gases have been pre-heated, less fuel is needed to heat the gases up to the turbine inlet temperature. Thus, by recovering some of the energy usually lost as waste heat, the recuperator can make a heat engine or gas turbine significantly more efficient.
It would be desirable to increase the fuel efficiency of a reaction-jet helicopter. Therefore, it is an object of this invention to provide a reaction jet helicopter with a recuperator that can deliver hot air to the turbine and to the rotor blades.